Why does my server slow and stop serving files?

I just setup a new server in my office running windows server 2003.  It is used as a file server, DNS, and DHCP.  Only third party software is symantec backup.  This is my first time to setup a server so I am very inexperienced.  This morning when I got to work I restarted the server for a fresh start, at this point all users (about 10) log on and work fine.  About two hours later the server for no apparent reason stops serving files.  When trying to access a file it may say access denied (like a permissions error) even though it has worked fine for the last two hours, or it just locks up when trying to open a file forcing a restart of the user's computer.  At this point the user's computer takes forever to log in (sometimes up to 20 minutes).  When this happens all users are affected at the same time, so I restart the server and they are able to work again for another couple of hours until this happens again.  Just to give you a little background I am an accountant not a computer guy although it is very much a hobby of mine.  This particular setup prior to the new server was just a bunch of PC's daisy chained together on a network that all shared files with each other.  The infrastructure (daisy chained network) is still in place which I know in itself is a problem but would this cause the server to malfunction as described above?  The files being server were on a standalone PC with a shared folder that all users could access.  This system actually worked pretty well, but for security and backup reasons we decided to get a server and try to build a decent small network.  Any ideas?